Storage library systems are used for storage and retrieval of data contained in cartridges stored in multiple cells within the library. At the core of the storage library system is a robotic assembly having a gripper for grasping the cartridges during moves between the cells and read/write drives. The cells of the library are usually arranged in a cylindrical pattern with the robotic assembly residing at the center. The robotic assembly includes a central column about which the rest of the assembly rotates to align with the desired column of cells. A carriage moves up and down the central column carrying a robotic hand to the selected cell. The robotic hand moves radially relative to the central column to insert and/or remove a cartridge from the selected cell.
Robotic assemblies commonly use velocity and/or relative position sensors to achieve the necessary alignment accuracy over the entire range of the storage library system at a reasonable cost. These sensors require the robotic assembly to be calibrated upon power-up and during reset operations. The calibration is accomplished by moving the robotic assembly to a known home position where a controller zeros the robotic assembly's absolute position. After the calibration, the controller can move the robotic assembly to the position of each cell in the storage library system.
In practice, no two storage library systems are alike. Mechanical tolerances, vibrations, settling effects, external stimulus, and other effects shift the actual position of the cells away from their theoretical or desired positions. Many systems accommodate these differences with vision subsystems that detect optical targets integrated into cell magazines. Feedback from the vision subsystem allows the robotic assembly to adjust for any alignment errors. The vision subsystem also read the bar codes from the cartridges allowing the controller to verify that the correct cartridge is in the cell being addressed.
Vision subsystems are expensive making them undesirable for small, low cost storage library systems. What is sought by storage library system manufactures is a low-cost mechanism to align the robotic assembly with the cells. The task of reading the bar codes on the cartridges may be allocated to the user to further reduce costs.